This invention relates to a system for detecting deterioration of a three-way catalyst arranged in an exhaust system of an internal combustion engine, based on output from an air-fuel ratio sensor arranged in the exhaust system at a location downstream of the three-way catalyst.
In general, in an internal combustion engine having a three-way catalyst arranged in the exhaust system thereof, when the engine undergoes transition from an open-loop control region in which the fuel supply to the engine is cut off as shown in FIG. 3 or an air-fuel mixture supplied to the engine is controlled to a lean state to a feedback control region in which the air-fuel ratio of the mixture is controlled to a theoretical value in a feedback manner, a post-catalyst air-fuel ratio sensor arranged in the exhaust system at a location downstream of the three-way catalyst continuously generates an output indicative of a lean air-fuel ratio over a predetermined time period after the above transition, due to oxygen storage effect which the three-way catalyst has, in spite of the fact that exhaust gases resulting from an air-fuel mixture having a nearly theoretical air-fuel ratio which is no longer lean start to be supplied to the three-way catalyst. Furthermore, the three-way catalyst has such a nature that the predetermined time period becomes shorter as the deterioration of the three-way catalyst advances.
In view of the above phenomenon and the nature of the three-way catalyst, systems have conventionally been proposed by Japanese Patent Publications (Kokai's) Nos. 1-203633, 1-211634, and 1-257738, in which a predetermined delaying time period for delaying the start of the feedback control based on the output from the post-catalyst air-fuel ratio sensor is provided after termination of the open-loop control, and at the same time the predetermined delaying time period is shortened as the deterioration of the three-way catalyst advances.
According to the above proposed systems, the degree of deterioration of the three-way catalyst is estimated based on total time over which the engine was in normal operating conditions, the number times of starting the engine, the travel distance of the vehicle, the difference between the maximum value and minimum value of output values from the post-catalyst air-fuel ratio sensor, the ratio between the period of an output signal from the post-catalyst air-fuel ratio sensor and the period of an output signal from a pre-catalyst air fuel ratio sensor arranged at a location upstream of the catalyst.
However, the detection of deterioration of the three-way catalyst in the above conventional systems is thus carried out in such an indirect manner, dependent on the estimated aging of the three-way catalyst, that it is impossible to accurately detect deterioration of the three-way catalyst.